There is known a hybrid work machine that uses not only an engine but also a generator motor for the purpose of achieving reduction of engine gas emissions and energy saving (refer to Patent Document 1). In the hybrid work machine of this type, the engine and the generator motor are provided coaxially, the generator motor and the engine drive a hydraulic pump, and hydraulic actuators are driven by a hydraulic fluid delivered from the hydraulic pump. When engine power is higher than pump absorption power, the generator motor is driven by excessive engine power to charge an electric storage device. When the engine power is lower than the pump absorption power, the generator motor is driven by electric power supplied from the electric storage device to assist the engine power. In this way, power of the generator motor makes up a shortfall in output power required by a machine body, so that the engine is miniaturized to improve fuel economy. There is also known that power of a generator motor makes up a shortfall in engine power during abrupt increase of power required by a hydraulic pump, for example, during abrupt operation (refer to Patent Document 2).